1. Field of the Invention
This invention is directed to the Direct Synthesis of organohalosilanes, and in particular, to the Direct Synthesis of organohalosilanes wherein a higher selectivity for the dialkyldihalosilanes is achieved.
2. Description of Related Art
The Rochow-Muller Direct Synthesis is the one-step preparation of organohalosilanes from copper-activated silicon and an organohalide. This reaction was disclosed in U.S. Pat. No. 2,380,995 to Rochow which issued Aug. 7, 1945, and in German Patent No. DE5348. The Direct Synthesis produces a mixture of silicon-containing products of the following general formulae: RSiX3, R2SiX2, R3SiX, R4Si, SiX4, HSiX3, RSiHX2, R2SiHX, and RnSi2X6−n, wherein R is a hydrocarbon, X is a halogen, and n is an integer less than or equal to 6.
Organosilanes such as methylchlorosilanes and phenylchlorosilanes are typically synthesized in fluidized bed reactors via the Direct Synthesis. Controlling and improving process performance to favor the formation of the dihalo moiety, R2SiX2, over the trihalo moiety, RSiX3, is an ongoing objective of researchers and manufacturers. The preference for the diorganodihalosilane is referred to as its selectivity. The selectivity is defined as the gravimetric ratio R2SiX2/RSiX3, abbreviated as D/T. Higher values are desirable. Oftentimes the selectivity is reported as the inverse ratio T/D, thus, a lower value would be desirable. Among the factors known to influence selectivity are choice of copper catalyst; identity, concentration, and ratio of promoters; composition of the silicon; reaction conditions; fluidization rate; and organohalide conversion.
Generally, fluidized bed reactors are used in commercial practice of the Direct Synthesis because they afford a good balance of gas-solid mass transfer at short contact times, good heat removal, high selectivity to dimethyldichlorosilane, and silicon conversions of about 80 to 95 wt. %. Yet, there is a need for more efficient heat removal and improved performance from Direct Synthesis reactors. Poor heat removal manifests itself as “hot spots” on which methyl chloride cracking occurs. Cracking ultimately leads to undesirable by-products such as methyltrichlorosilane, methyldichlorosilane, and trichlorosilane which diminish the formation and selectivity to dimethyldichlorosilane. Thus, more efficient heat removal and/or elimination of hot spots would improve selectivity to the desired dihalo product, R2SiX2.
Slurry reactors for the Direct Synthesis of organohalosilanes can provide better results than the prior art fluidized bed reactors. In a slurry reactor, catalytically activated silicon particles are suspended in a thermally stable, high boiling heat transfer medium wherein the reaction with the organohalide occurs at an elevated temperature. This type of reactor is taught in U.S. Pat. No. 3,505,379 to Bonitz et al. which issued on Apr. 7, 1970, U.S. Pat. No. 3,641,077 to Rochow which issued on Feb. 8, 1972, U.S. Pat. No. 3,775,457 to Muraoka et al. which issued on Nov. 27, 1973, and U.S. Pat. No. 5,728, 858 to Lewis et al. which issued on Mar. 17, 1998, and assigned to the assignee of the present invention.
German Patent No. DE887343 teaches that silicon and copper powders may be dispersed in liquid paraffin and reacted with methyl chloride to yield methylchlorosilanes. Copper usage was 10 wt. % based on a weight of silicon charged into the reactor.
German Patent Application No. DE 1100006, and German Patent Nos. DE1161430 and DE1132901 teach the preparation of chlorosilanes, methylchlorosilanes and ethylchlorosilanes from the reaction of the corresponding alkyl halide with so-called “active silicon” and ferrosilicon in a liquid paraffin slurry at 180° C. to 200° C. The “active silicon” was made by the action of chlorine on calcium disilicide. No copper was used in some of the experiments. Other solvents used include silicone oils, high boiling polychlorosilanes, and alkylsilicates. The results are summarized in Bonitz, E., Angewandte Chemie. International Edition, Vol. 5, No. 5, pp. 462–469 (1966).
German Patent No. DE1079607 discloses a process for slurry-phase activation of silicon and silicon alloys with copper. However, the copper source, such as copper (II) acetylacetonate, must be soluble in the reaction solvent. Solvents used include paraffins, silicate esters, and alkylchloropolysilanes.
German Patent No. DE920187 prefers the use of molten salts as solvents in slurry reactors for the Direct Synthesis of organohalosilanes.
In the fluidized bed Direct Synthesis of organohalosilanes, copper and copper salts, chlorides, and oxides, are used as catalysts to activate the silicon. Typically, the copper catalysts have particle sizes of about 1 to 10 microns and are considerably smaller than those of the silicon particles. Solid promoters of similar particle size are used in the Direct Synthesis to enhance selectivity to the dialkyldihalosilane. Therefore, fluidization velocity is important so that the largest silicon particles are suspended in the organohalide gas stream. However, this fluidization velocity oftentimes exceeds the escape velocity of the smallest copper catalyst and promoter particles from the reactor bed. As a result, the copper catalyst and promoters are rapidly elutriated from the reactor and their consumption is increased, thereby increasing operational costs.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of making organohalosilanes using the Direct Synthesis with greater selectivity to the dialkyldihalosilane.
It is another object of the present invention to provide a method of making organohalosilanes using the Direct Synthesis with more efficient heat removal and/or elimination of hot spots.
A further object of the invention is to provide a method of making organohalosilanes using the Direct Synthesis with lower amounts of catalyst and promoters.
It is yet another object of the present invention to provide a composition useful in the method of making organohalosilanes using the Direct Synthesis.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.